A typical such fluid-powered impact mechanism is provided with a hammer piston that can be moved back and forth by means of a controller, and a guide unit on which the impact mechanism is carried. Furthermore, the impact device is provided with a control valve, designed as pressure-limiting valve (PSOV) or shutoff valve automatically deactivating the impact mechanism if the working pressure caused by the inlet pressure exceeds a predetermined maximum value/peak value by either blocking the pressure line or stopping the controller in one of its end positions, that is either in the position of the working stroke or of the return stroke. Finally, the impact mechanism is provided with a hydraulic stop buffer for decelerating the hammer piston when a predetermined impact area is passed.
The above-described device is known from EP 0 934 804 A2 (U.S. Pat. No. 6,959,967).
Fluid-powered impact devices, particularly those serving for milling stones, concrete or other construction materials are mostly used as additional or attached devices for construction machines such as excavators, loader or other carrier units. The connection of a impact device to a boom of a hydraulic excavator and the supply of the impact device by means of a pressure line as well as a return line are already described in DE 40 36 918 A1 (U.S. Pat. No. 5,174,387). The guide unit carrying the impact mechanism can be designed as a housing (hammer box) or as a supporting frame. The impact device consists of a cylinder in which a hammer piston is guided, a cylinder cover and a lower part of the hammer in which the chisel or the insertion end is mounted by means of wear is bushings.
The hammer piston is designed as a differential piston, i.e. it is provided with two oppositely directed annular actuator faces of different sizes. The lower actuator face, by means of which the return stroke is triggered when a pressurization takes place, is continuously pressurized with a predetermined operating pressure. The upper actuator face, by means of which the advance stroke is initiated by pressurization, is pressurized with the operating pressure or relieved to the sump pressure depending on the position of the spool valve. The advance stroke can be realized, since the upper annular actuator face is larger than the other, thus, pressurization with the operating pressure results in a force acting in striking direction. During the so-called advance stroke, the moving piston displaces the oil displaced by the smaller annular actuator face toward a chamber above the larger upper annular surface, which is also pressurized with the oil coming from the pump. During the return stroke, the oil from the pump flows only in the direction of the actuator face with smaller dimensions, whereas the oil from the actuator face with larger dimension is discharged by means of a throttle or an orifice providing an equilibrated operation of the hammer.
In particular, the impact mechanisms mentioned here are provided with a gas buffer, namely a chamber under gas pressure, into which the upper end face of the piston engages. The gas pressure in the chamber acts as an additional force on the piston in direction of the advance stroke. The part of the piston positioned at the other end of the piston, including the end face there or the striking surface reaches into a so-called striking chamber that is connected to the atmosphere.
Depending on the actuation position, the spool valve mentioned above which is preferably positioned in the cover either connects the actuator face with larger dimensions to the supply line such that the operating pressure is applied to it or during the return stroke depressurizes the surface by means of a line connecting the return line to the sump.
In addition, the spool valve of the control valve can be provided with a piston with two actuator faces, one of the surfaces or partial surfaces being constantly pressurized with a supply line pressure and the other surface being optionally either pressurized with or relieved of the supply line pressure; in the latter event, a connection to the sump is opened. Thanks to the different sizes of the actuator faces, the spool valve can be moved into one of its end positions.
The pressure-limiting valve or pressure-relief valve described in EP 0 934 804 A2 is connected to the pressure line pressurized with the working pressure and automatically deactivates the impact mechanism if the working pressure exceeds a predetermined peak value created by the operating pressure, by blocking either the pressure line or the controller in one of its end positions, namely the full-forward or the full-rearward position. Thus, it is guaranteed that the impact device is not exposed to inadmissibly high forces.
If the chisel does not engage the material to be broke up or if the chisel penetrates deeply into the material when a stroke is carried out, the piston passes its predetermined (theoretical) stroke impact area in the direction of the advance stroke and after a certain overtravel penetrates with its lower actuator face or the lower large-diameter portion, into a hydraulic stop buffer decelerating the piston before it can hit the lower part. This way, the impact on the components is reduced and damages are avoided.
The theoretical impact area describes the area where the lower front surface of the piston touches the upper back face of the chisel when the chisel is positioned at the abutment, i.e. in the theoretical impact position. Passing the theoretical impact area means the piston is positioned such that the lower end face of the piston is positioned below or above (during the return stroke) (during the return stroke) the theoretical impact area.
The pressure line can be blocked by the control valve or the controller can be stopped in one of its end positions as a preventative measure to avoid damage; for if the operating pressure is too high, the piston is accelerated too much and thus the level of stroke energy becomes too high. The above-described embodiment, however, has the following disadvantage: If the chisel does not contact the material to be destroyed or if the chisel penetrates (too) deeply into the material when advancing, the piston passes its theoretical impact area to a certain extent and penetrates into the hydraulic stop buffer with its lower actuator face or the large-diameter portion. In order to move the piston rearward out of the buffer, the hydraulic medium has to get into the chamber is below the actuator face with smaller surface by means of a supply line. Due to the piston passing the theoretical impact area, the hydraulic medium can only flow through a small gap between the lower large-diameter portion and the cylinder bore. The gap represents a comparatively high resistance in the sense of a throttle, by means of which the pressure in the pressure line connected to the annular chamber mentioned above is increased and thus reaches a level exceeding the level of operating pressure allowed, which again results in the pressure-limiting valve being actuated. This means that the hydraulic hammer is unintendedly switched off when the hammer piston is lifted.